Long Term Evolution (LTE) uses orthogonal frequency division multiplexing (OFDM) in the downlink and discrete Fourier Transform (DFT)-spread OFDM in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid 2 as illustrated in FIG. 1, where each resource element 4 corresponds to one OFDM subcarrier during one OFDM symbol interval.
In the time domain, LTE downlink transmissions, as shown in FIG. 2, are organized into radio frames of 10 ms, each radio frame 6 consisting of ten equally-sized subframes of length Tsubframe=1 ms.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks (RB), where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
The notion of virtual resource blocks (VRB) and physical resource blocks (PRB) has been introduced in LTE. The actual resource allocation to a user equipment (UE) is made in terms of VRB pairs. There are two types of resource allocations, localized and distributed. In the localized resource allocation, a VRB pair is directly mapped to a PRB pair, hence two consecutive and localized VRB are also placed as consecutive PRBs in the frequency domain. On the other hand, the distributed VRBs are not mapped to consecutive PRBs in the frequency domain; thereby providing frequency diversity for data channel transmitted using these distributed VRBs.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information about which UEs data is transmitted and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI). The downlink subframe also contains common reference symbols (CRS), which are known to the receiver and used for coherent demodulation of, e.g., the control information. A downlink system 8 with CFI=3 OFDM symbols for control signaling is illustrated in FIG. 3.
Carrier Aggregation
The LTE Rel-10 specifications have recently been standardized, supporting Component Carrier (CC) bandwidths up to 20 MHz (which is the maximal LTE Rel-8 carrier bandwidth). Hence, an LTE Rel-10 operation wider than 20 MHz is possible and appears as a number of LTE carriers to an LTE Rel-10 UE.
In particular for early LTE Rel-10 deployments it can be expected that there will be a smaller number of LTE Rel-10 capable UEs compared to many LTE legacy UEs. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy UEs, i.e., that it is possible to implement carriers where legacy UEs can be scheduled in all parts of the wideband LTE Rel-10 carrier. The straightforward way to obtain this would be by means of Carrier Aggregation (CA). CA implies that an LTE Rel-10 UE can receive multiple CC, where the CC have, or at least the possibility to have, the same structure as a Rel-8 carrier. CA is illustrated in FIG. 4 which shows five 20 MHz bandwidths 10 and the aggregated bandwidth 12 of 100 MHz.
The Rel-10 standard supports up to 5 aggregated carriers where each carrier is limited in the radio frequency (RF) specifications to have a one of six bandwidths namely 6, 15, 25, 50, 75 or 100 RBs (corresponding to 1.4, 3, 5, 10, 15, and 20 MHz respectively).
The number of aggregated CCs as well as the bandwidth of the individual CC may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CCs in downlink and uplink is the same whereas an asymmetric configuration refers to the case that the number of CCs is different. It is important to note that the number of CCs configured in the network may be different from the number of CCs seen by a UE. A UE may, for example, support more downlink CCs than uplink CCs, even though the network offers the same number of uplink and downlink CCs.
During initial access a LTE Rel-10 UE behaves similarly to a LTE Rel-8 UE. Upon successful connection to the network a UE may, depending on its own capabilities and the network, be configured with additional CCs in the UL and DL. Configuration is based on radio resource control (RRC). Due to the heavy signaling, and the rather slow speed of RRC signaling, it is envisioned that a UE may be configured with multiple CCs even though not all of them are currently used. If a UE is activated on multiple CCs this would imply it has to monitor all downlink (DL) CCs for physical downlink control channel (PDCCH) and physical downlink shared channel (PDSCH). This implies a wider receiver bandwidth, higher sampling rates, etc., resulting in high power consumption.
Component Carrier Types
Initially, the user equipment will be configured with one UL/DL pair of component carriers, on which it made the initial random access. These component carriers are together called the Primary Cell (PCell).
The uplink (UL) PCell is configured with Physical Uplink Control Channel (PUCCH) and used for transmission of Layer 1 (L1) uplink control information. This also includes (Channel State Information) CSI for the DL transmission on the activated SCells.
In addition to the PCell, the base station may configure the user equipment with additional serving cells, so called “Secondary Cells” (SCells) as extra resources when needed. The user equipment may be configured with one or more, up to four SCells.
Random Access
In LTE, as in any communication system, a UE may need to contact the network (via the eNodeB) without having a dedicated resource in the Uplink (from UE to base station). To handle this, a random access procedure is available where a UE that does not have a dedicated UL resource may transmit a signal to the base station. The first message of this procedure is typically transmitted on a special resource reserved for random access, a physical random access channel (PRACH). This channel can for instance be limited in time and/or frequency (as in LTE). FIG. 5 illustrates random access preamble transmission 14 including uplink resources used for data transmission 16 and uplink resources reserved for random access preamble transmission 18. The resources available for PRACH transmission are provided to the UEs as part of the broadcasted system information (or as part of dedicated RRC signaling in case of, e.g., handover).
In LTE, the random access procedure can be used for a number of different reasons including:                Initial access (for UEs in the LTE IDLE or LTE DETACHED states)        Incoming handover        Resynchronization of the UL        Scheduling request (for a UE that is not allocated any other resource for contacting the base station)        Positioning        
The contention-based random access procedure 20 used between a UE 22 and an LTE RAN 24 in LTE is illustrated in FIG. 6. Initially, the UE 22 receives system information for random access 26 from the LTE RAN 24. The UE 22 starts the random access procedure by randomly selecting one of the preambles available for contention-based random access as shown in block 27. The UE 22 then transmits the selected random access preamble on the PRACH to an eNode B in the LTE RAN 24 in step 28.
The RAN acknowledges any preamble it detects by transmitting a random access response (MSG2) 30 including an initial grant to be used on the uplink shared channel, a temporary cell-radio network temporary identifier (C-RNTI), and a time alignment (TA) update based on the timing offset of the preamble measured by the eNodeB on the PRACH. The MSG2 30 is transmitted in the DL to the UE 22 and its corresponding PDCCH message CRC is scrambled with the RA-RNTI.
When receiving the response the UE 22 uses the grant to transmit a message (MSG3) 32 that in part is used to trigger the establishment of radio resource control and in part to uniquely identify the UE 22 on the common channels of the cell. The timing alignment command provided in the random access response is applied in the UL transmission in MSG3 32. The eNB can change the resources blocks that are assigned for a MSG3 32 transmission by sending an UL grant the CRC of which can be scrambled with the temporary cell-radio network temporary identifier (TC-RNTI).
The MSG4 34 which is then contention resolution has its PDCCH CRC scrambled with the C-RNTI if the UE previously has a C-RNTI assigned. If the UE does not have a C-RNTI previously assigned its PDCCH CRC is scrambled with the TC-RNTI.
The procedure ends with the LTE RAN 24 solving any preamble contention that may have occurred for the case where multiple UEs transmitted the same preamble at the same time. This can occur since each UE randomly selects when to transmit and which preamble to use. If multiple UEs select the same preamble for the transmission on the RACH, there will be contention between these UEs that needs to be resolved through the contention resolution message (MSG4) 34. The case when contention occurs is illustrated in FIG. 7, where two UEs, UE1 22 and UE2 36, transmit the same preamble, p5 38, at the same time. A third UE, UE3 40, also transmits at the same RACH, but since it transmits with a different preamble, p1 42, there is no contention between UE3 40 and the other two UEs (UE1 22 and UE2 36).
The UE 22 can also perform non-contention based random access. A non-contention based random access or contention free random access can, e.g., be initiated by the eNB to get the UE 22 to achieve synchronisation in UL. The eNB initiates a non-contention based random access either by sending a PDCCH order or indicating it in an RRC message. The latter of the two is used in case of handover (HO).
The eNB can also order the UE 22 through a PDCCH message to perform a contention based random access, the procedure for this is illustrated in FIG. 7. The procedure for the UE 22 to perform contention free random access is illustrated as shown in FIG. 8. The UE 22 receives system information for random access 44 and a random access order 46 from the LTE RAN 24. The UE 22 then transmits a random access preamble to the LTE RAN 24. Similar to the contention based random access the MSG2 30 is transmitted in the DL to the UE 22 and its corresponding PDCCH message CRC is scrambled with the RA-RNTI. The UE 22 considers the contention resolution successfully completed after it has received MSG2 50 successfully.
For the contention free random access, as for the contention based random access, the MSG2 50 contains a timing alignment value. This enables the eNB to set the initial/updated timing according to the UEs transmitted preamble.
In LTE in Rel-10 the random access procedure is limited to the primary cell only. This implies that the UE 22 can only send a preamble on the primary cell. Further MSG2 30 and MSG3 32 is only received and transmitted on the primary cell. MSG4 34 can however in Rel-10 be transmitted on any DL cell.
In LTE Rel-11, the current assumption is that the random access procedure will be supported also on secondary cells, at least for the UEs supporting Rel-11 carrier aggregation. So far only network initiated random access on SCells is assumed.